Acid fog, composed chiefly of H2SO4 and HNO3, occurs on a seasonal basis in Los Angeles and elsewhere in the country. The following physicochemical properties of acid fog have been reported: pH as low as 1.7, total acid mass in excess of 100 ug/m3, hypo-osmolar liquid water content of about 0.2 g/m3, and a mass median diameter of the fog droplets on the order of 15 um. We hypothesize that these fog droplets are irritating to the respiratory system. Our specific aims are to generate and characterize H2SO4-fog for the acute exposure of human volunteers. The fog will be administered in a small-volume head dome. Two groups of adults will be studied: healthy non-smokers and asthmatics. The functional end-points will include: airway reactivity, maximal expiratory flow-volume curves, respiratory flow resistance, and bronchopulmonary deposition and clearance of a radiolabeled aerosol. The possible roles of post-ganglionic nerve stimulation and mast cell mediator release in the response to acid fog will be studied.